Class of Threads?

Discussion in 'Gunsmithing' started by Badger300wm, Mar 16, 2014.

  1. Badger300wm

    Badger300wm Well-Known Member

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    looking for information on class of threads and how they work. Specifically I want to take off a remington barrel and put a bartlein barrel with the same threads and pitch but different class of threads on it. Honestly I dont know which either of them have and have just been told that the new barrel has a different class of threads than the remington, thank you.
     
  2. Trickymissfit

    Trickymissfit Well-Known Member

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    there are different grades of threads, and there is also what you get as compared to the thread diameter. Nobody here is grinding the helix angle on barrel threads, and there's only a small handful of folks capable of grinding an internal thread (I've only known about six with the knowledge). Turning the thread in a lathe is good for a class three at best, but you still can get a nice thread that way. You just match the O.D. pitch diameter to the I.D. pitch diameter. Some go so far as to lap the two into each other. The Federal standards for thread fit also figure in thread contact area. With a typical one inch plus thread your talking 65% max. Much more and the threads will guall on contact, and will not come apart very well. There's a lot of good reading on this in the Machinist Hand Book, and the Federal Standards book (can get both at the library).

    The threads in the receiver should be used as a master to cut the male thread to. You must make them as strait as possible before even thinking about turning the O.D.
    gary
     

  3. Canadian Bushman

    Canadian Bushman Well-Known Member

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    Tricky pretty much nailed it.

    A class of thread basically is just a spec for tolerances, 1 being the loosest ascending to tighter tolerances. IMHO even a class 3 thread allows for more slop than i would prefer for a gun barrel.

    Re-iterating what tricky said, class of thread has no bearing if you cut your receiver oversize truing the factory threads. This is only remedied by cutting the barrel threads to match the receiver. Which is S.O.P. for most gunsmiths.

    Ive heard of smithy's cutting threads oversize and lapping them in, but ive never seen any actually do it. I would focus on getting the back wall of the barrel tenon perpendicular to the barrels bore, and the actions lugs and face perpendicular to its bore. The actions face and the barrel tenon's back wall will ultimately determine the direction the slop in the threads is removed when the barrel is torqued. Assuming the faces of the recoil lug are parallel.

    +1 on machinist handbook.
    More than you ever wanted to know about threads in there.
     
  4. Badger300wm

    Badger300wm Well-Known Member

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    Thanks for the replies guys, me cave man no spekkum big words. I guess my main concern is will it be neccesary to recut the barrel threads to match the "class" on the reciever or does it matter as long as they are the same thread pattern? (16 tpi x 1.062). If they do need recut does anyone know the class on a standard remington short action?
     
  5. Canadian Bushman

    Canadian Bushman Well-Known Member

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    I dont think you are following what me and tricky are saying.

    A class 1,2, or 3 male thread will all thread into a class 1,2, or 3 female.

    By specifying a class you are limiting the amount of slop that it will have when mated to its opposing part.

    Class 1 is the sloppiest, and class 3 are the tightest. At least the tightest i ever cut.

    If both your barrel and action are cut perfectly nominal at a class 3 fit, they will still be much sloppier than if you had a barrel blank cut to the actual pitch diameter that exist in your particular action.

    Anyone who has dealt with enough factory actions will tell you, rarely are they perfectly nominal, or even square to the bolt race.
    This is why smiths barrel an action the way they do.
     
  6. Trickymissfit

    Trickymissfit Well-Known Member

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    if you remove the barrel and shine a bright light into the female threads, you will often see a line running lengthways to the thread. That's the parting line from their tap. Nothing wrong with a tap except it's only as good as the bore was cut (often another operation), and the alignment of the fixture. But if you cut the bore and threads as one operation you will normally see a straighter thread. This puts everything into the same relationship. In otherwords the thread pitch diameter is concentric with the I.D. of the bore.
    gary